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Abstract:

A polymerizable ligand comprising, in one embodiment, a polyaromatic
compound, with a terminal functional group, non-covalently bonded to the
sidewalls of carbon nanotubes. This structure preserves the structural,
mechanical, electrical, and electromechanical properties of the CNTs and
ensures that an unhindered functional group is available to bond with an
extended polymer matrix thereby resulting in an improved polymer-nanotube
composite.

Claims:

1. A method for providing a polymerizable functionality to carbon
nanotubes (CNTs), the method comprising the step of non-covalently
bonding a polymerizable ligand to the CNTs.

2. The method of claim 1, wherein the polymerizable ligand comprises a
polyaromatic molecule with a polymerizable group attached thereto.

19. The polymer-nanotube composite as recited in claim 11, wherein the
polymerizable ligand is bonded to the sidewalls of the CNTs.

20. The polymer-nanotube composite as recited in claim 11, wherein the
polymerizable ligand is also bonded covalently to the CNTs.

21. A polymer-nanotube composite comprising:carbon nanotubes (CNTs)
functionalized by having vinylanthracene non-covalently bonded to the
sidewalls thereof; andnylon bound to the functionalized CNTs.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the benefit of prior filed, co-pending U.S.
provisional application: Ser. No. 60/823,769, filed on Aug. 29, 2006,
which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

[0003]1. Field of the Invention

[0004]The present invention relates generally to functionalizing nanotubes
to form improved polymer-nanotube composites and, more particularly, to a
method for functionalizing carbon nanotubes with a polymerizable ligand
formed using polymerizable groups, for example, vinyl or styryl groups,
in combination with a polyaromatic molecule such as a polyaromatic
hydrocarbon (PAH) for the purpose of achieving significant improvements
in the properties of a polymer-nanotube composite due to improved
dispersion and chemical bonding between the polymer matrix and the
nanotube itself.

[0005]2. Description of Related Art

[0006]Sustaining habitation on the moon, or any other planet, will require
light weight, high strength structures that protect against both
radiation and particulates. Chemical synthesis techniques to enable
strong bonding between carbon nanotubes and polymers to form improved
polymer-nanotube composites, technology which could be used for this
application, are needed.

[0007]Polymer properties such as electrical conductivity have been shown
to be enhanced by incorporating therein a combination of carbon fibers or
carbon nanotubes (CNTs). Additionally, CNTs have been shown to prevent
delamination and provide structural stability in polymer composites.
Because CNTs have uniquely high strength to mass ratio, intrinsic light
weight, thermal conductivity, electrical conductivity, and chemical
functionality, and, as noted, have been shown to prevent delamination and
provide structural stability in polymer composites, they can impart these
properties to polymers when effectively combined therewith.

[0008]Though CNTs have extraordinary mechanical properties, their ability
to strengthen polymers and epoxies is limited by the strength of
interfacial bonding. As a result, when incorporated into polymeric resin
without cross-linking or functionalization, they lack the ability to
transfer loads across the structure.

[0009]CNTs can be functionalized via covalent or non-covalent bonding, to
either the ends of the nanotubes or to the sidewalls. Covalent
functionalization often requires beginning with modified tubes, such as
fluorinated nanotubes, or with purified tubes where defect sites in the
CNTs are produced by oxidation. Because these modifications often result
in the disruption of the bonds along the tubes themselves, covalent
functionalization can degrade the mechanical and electrical properties of
the nanotubes and, thus, is not ideal for all applications.

[0010]Therefore, the present invention has been made in view of the above
problems, and it is an objective of the present invention to provide a
method for functionalizing carbon nanotubes using polymerizable ligands
and to form improved polymer-nanotube composites utilizing the
functionalized nanotubes.

SUMMARY OF THE INVENTION

[0011]Non-covalent functionalization to the sidewalls of CNTs can be
attained by exploiting the van der Waals and pi-pi bonding between the pi
electrons of the CNTs and that of a polyaromatic molecule, for example, a
polyaromatic hydrocarbon (PAH) such as anthracene. This type of
functionalization results in higher degrees of functionalization as the
entire length of the CNT can be functionalized rather than just the ends
and specific active sites. Like end-functionalization, non-covalent
functionalization also opens up the possibility for tailoring the
functionalization via the choice of molecule.

[0012]For the purpose of polymerizing the CNT to a polymer resin or epoxy,
in one embodiment, a polymerizable ligand comprising a polyaromatic
molecule such as PAH with an appropriate polymerizable group such as a
vinyl, styryl, or amino group can be non-covalently bonded to the CNTs.
In the embodiment shown in FIG. 1, single-walled carbon nanotubes (SWNTs)
are functionalized with a polymerizable ligand, vinylanthracene, thereby
enabling improved crosslinking or bonding and dispersion of CNTs into a
polymer. This results in improving the mechanical properties of the
interface between the CNTs and the polymer thereby imparting many of the
valuable properties of CNTs into the polymer matrix resulting in a
significantly improved polymer-nanotube composite.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]These and other objects, features and advantages of the invention
will be apparent from a consideration of the following Detailed
Description considered in conjunction with the drawing Figures, in which:

[0014]FIG. 1 is a schematic of a SWNT functionalized with vinylanthracene.

[0015]FIG. 2 is a graph illustrating the absorption spectra of SWNT
solution before and after functionalization with vinylanthracene.

[0017]FIG. 4 is a scanning electron microscope (SEM) image of nylon 12
with approximately 1% by weight of purified but non-functionalized
multi-walled carbon nanotubes (MWNTs).

[0018]FIG. 5 is an SEM image of nylon 12 approximately 1% by weight of
purified but non-functionalized MWNTs.

[0019]FIG. 6 is an SEM image of nylon 12 approximately 1% by weight of
vinylanthracene-functionalized MWNTs.

[0020]FIG. 7 is an SEM image of nylon 12 with approximately 1% by weight
of vinylanthracene-functionalized MWNTs.

[0021]FIG. 8 is an SEM image of nylon 12 with approximately 1% by weight
of vinylanthracene-functionalized MWNTs.

DETAILED DESCRIPTION

[0022]In a first set of experiments, the CNTs were SWNTs produced by the
HipCO method and purified by refluxing in 3M HNO3 for 16 hours. To
functionalized the nanotubes, a mixture of CNTs and vinylanthracene, at a
ratio of 1:2 by weight, were sonicated for approximately one hour in dry
tetrahydrofuran (THF). To remove unreacted vinylanthracene, CNTs were
collected by filtration, washed with THF, and dried over vacuum in air.
Absorbance and fluorescence data were collected as evidence of
functionalization.

[0023]The CNTs in the second set of experiments were MWNTs purified in 3M
HNO3 for 4 hours. A shorter reflux time was chosen to preserve
longer length tubes. To functionalize the nanotubes, a mixture of CNTs
and the appropriate anthracene derivative, at a ratio of 2:5 by weight,
were mixed in THF for 72 hours. To remove unreacted ligand, CNTs were
collected by filtration, washed with THF, and dried over vacuum in air.
Fluorescence data was collected as evidence of functionalization.

[0024]In situ synthesis of nylon 12 was completed in solution, based on
the Haggenmueller method for nylon 6. A solution of 0.0325M
diaminododecane was prepared by stirring for 24 hours in chloroform,
solution A. A solution of dodecanedioyl chloride in toluene was prepared
at three times the concentration, 0.0975M, solution B. To prepare a batch
of Nylon 12, 300 mL of Solution A were placed in a 600 mL beaker and
stirred with an overhead stirrer at 230 rpm for about one minute. If
appropriate, CNTs from above were ground with mortar and pestle, and then
added to the beaker. Finally, 100 mL of solution B were added and left to
stir for 30 minutes. The nylon produced was collected with a glass frit
filter, washed with toluene and chloroform, and dried over vacuum in air.
Typical yields were about 85%.

[0025]Nylon produced from the in situ methods was analyzed by scanning
electron microscope (SEM). The polymer was overcoated with gold to
improve the quality of the images recorded. The SEM used was Hitachi
S-4700 Cold Field Emission Scanning Electron Microscope (FE-SEM). Images
were recorded at 5 kV at short working distances around 5 to 6 mm.

[0026]Electronic structure of carbon nanotubes is dominated by van Hove
singularities which give rise to distinct peaks in the density of states
as seen by I/V measurements. The first electronic transition in metallic
carbon nanotubes is denoted M11 and the first and second electronic
transitions in semiconducting carbon nanotubes are denoted S11 and
S22, respectively. These transitions typically correspond to
absorbances in the visible and near infrared wavelengths. Absorption
spectra of solution-phase carbon nanotube suspensions typically exhibit
all of these features due to the presence of all types of CNTs in
solution. Chemical affinity-induced separation of metallic from
semiconducting nanotubes via functionalization and centrifugation has
been tracked via these resonances, as has chemical doping resulting in
disruption of the electronic structure of the CNTs. Changes in these
resonances were used to observe changes in electronic structure resulting
from functionalization with vinylanthracene.

[0027]Surface modification has been shown to affect the S11 and
S22 electronic transitions in carbon nanotubes. A solution having
molarity 2×10-3M of purified unfunctionalized nanotubes
(background substracted for 1% wt. Triton X-100 in THF) exhibited broad
peaks characteristic of solution-phase S11 and S22 level
transitions. Functionalized nanotube solution exhibited no such peaks
within the same range of wavelengths, as shown in FIG. 2. This diminished
S11 peak intensity after non-covalent functionalization of nanotube
sidewalls is known in the literature. It is thought that complexation in
this manner may change the electronic density of states of the nanotube.
Similarly functionalized SWNTs have been compared to highly defective
double-walled nanotubes having significantly different density of states
from a pristine SWNT.

[0028]In addition to changes in the absorption spectra, functionalization
of carbon nanotubes can be observed through changes in fluorescence
before and after functionalization, particularly when the ligand or
attached group is fluorescent. This is the case with vinylanthracene,
which fluoresces strongly at ˜400-420 nm when excited at 350 nm, as
shown in FIG. 3. The SWNTs used in these experiments did not have any
overlapping fluorescence in this region, thus the presence of
vinylanthracene fluorescence peaks in a well-rinsed and purified
functionalized SWNT sample is indicative of functionalization.
Fluorescence spectra of purified SWNTs prior to functionalization with
vinylanthracene, of vinylanthracene in solution, and of
vinylanthracene-functionalized SWNTs are shown in FIG. 3.

[0029]Precipitated CNTs were removed from filter paper after washing and
suspended in heptane. Heptane was selected because it is a dry solvent
that does not have any fluorescent or Raman peaks near those of
vinylanthracene. Emission spectrum of this solution and of a
2×10-4M standard of vinylanthracene in heptane were recorded.
The wavelengths of the fluorescence peaks were very similar in the two
cases. Peaks appeared at 404 and 423 nm for the unbound vinylanthracene,
and at 403 and 422 nm for the filtrate solution. By contrast, the
relative heights of the peaks shifted somewhat; in the standard sample,
the peak at 422 nm is higher than that at 403 nm, while the peaks are
approximately equal in height for the functionalized CNTs. This
difference may be the result of a slight fluorescence quenching due to
energy transfer between the bound vinylanthracene and CNT wall.

[0030]A series of standards of vinylanthracene were used to create a
calibration curve. The estimated concentration of CNT-bound
vinylanthracene using this calibration curve is approximately
3.2×10-6M. The number of vinylanthracene molecules per SWNT
can be computed if the average molecular weight of a carbon nanotube is
known. For these calculations, the weight of the CNTs was estimated by
assuming an average bond length of 0.32 nm, a tube diameter of 0.7 nm,
and tube lengths between 1 and 10 nm. Assuming the average molecular
weight of a carbon nanotube is 2,242,800 g/mol, the ratio is roughly
estimated to be about 143 molecules of vinylanthracene per nanotube. A
similar set of fluorescence experiments was completed to confirm the
functionalization of MWNTs with vinylanthracene.

[0031]Scanning electron microscopy (SEM) was used to image nylon that was
produced by the in situ process in order to determine how well the CNTs
were dispersed throughout the polymer. It is critical that the CNTs be
well dispersed in order to maximize their effect on the material's
mechanical and rheologic properties. If the tubes are clumped together,
then large regions of the polymer substrate will insulate the tubes, and
the overall thermal and electrical conductivity will be low. In addition,
evidence of good dispersion may indicate that the polymer is chemically
bonding to the functional groups along the CNTs. It has been suggested
that CNTs which are dispersed within a polymer but not chemically bound
to it, can rotate or shift, and therefore do not effectively resist
strain applied to the material. Creating a physical bond directly between
the polymer and the CNTs or its functional groups, should increase the
modulus of the bulk material.

[0032]The nylon 12 polymer in FIGS. 4-8 was made by the in situ method
described above. Purified MWNTs were added to the first batch at about 1%
by weight, assuming a 100% yield of polymer. The second batch of nylon
was made with the same percentage of MWNTs, however these tubes were
functionalized with vinylanthracene. As shown in FIGS. 4 and 5, it is
obvious that the non-functionalized tubes are collected in a single large
clump which is surrounded entirely by polymer. However, the
functionalized CNTs in the second batch appear in FIG. 6 to be dispersed
throughout the polymer. At the 1 micron scale in FIG. 7, it is possible
to see individual tubes separated from one another by nylon polymer.

[0033]In the FIG. 8, there are small nodules of material that appear to be
placed along the length of a long tube. This image indicates beads of
nylon 12 growing at reactive sites along the CNT. The reactive sites can
include defects in the CNT itself, including carboxylic acid sites, or
non-covalently attached functional groups.

[0034]CNTs functionalized with vinylanthracene appear to aid in the
dispersion of MWNTs through the nylon 12 polymer matrix. Though not
directly involved in the chemical reaction between the amine and the
dioyl chloride of the polymer, the vinyl group may interact with the
polymer as well. The electron cloud surrounding the double bond of the
vinyl group may share some electron density with the pi electrons in the
amide link of the polymer. The location of the vinyl group, hanging off
of the anthracene molecule with little steric interference from other
bonds, may increase the likelihood that the CNT becomes entangled in the
polymer matrix.

[0035]The polymerization of MWNTs with nylon 12 where the MWNTs have been
functionalized with different anthracene derivatives is also possible.
Candidates include diamino anthracene or dioyl chloride anthracene. In
general, however, materials containing functionalized CNTs will see
significant improvements due to improved dispersion and the chemical
bonding between the polymer matrix and the nanotube itself.

[0037]The polymer-nanotube composites of the present invention may be
used, for example, as thermoplastics, thermosets and conductive fillers.
These materials may, for example, be used to protect sensitive electronic
devices against the threat of electrostatic discharge and electromagnetic
or radio frequency (RF) interference. In addition, these materials may be
used to create paint that is applied, for example, to the walls of homes,
commercial properties, or automobile body parts. The polymer-nanotube
composites of the present invention may be used to create electrostatic
materials, electromagnetic shielding, active electronics, printed circuit
boards or conducting adhesives

[0038]The methods of the present invention may be used to create
biocompatible carbon-nanotube polymers. The methods of the present
invention may be used to create carbon-nanotube polymers that are
incorporated into plastic chips, a wide variety of consumer products, or
electronic devices. The methods of the present invention may be used to
create carbon-nanotube polymers that are incorporated into rechargeable
batteries, solid ectrolytes, electrical displays, photovoltaics,
actuators, switches, sensors (for example, chemical, biochemical or
thermal sensors), or smart structures.

[0039]The methods of the present invention may be used to create light
weight, high strength structures. These structures may, for example,
protect against radiation and particulates. Light weight, high strength
structures created according to the methods of the present invention may
be used, for example, to create vehicles, including aircraft and
spacecraft, as well as sustaining habitation, hospitals, or other
buildings on the moon, earth, or any other planet.

[0040]While various embodiments have been described above, it should be
understood that they have been presented by way of example only, and not
limitation. Thus, the breadth and scope of a preferred embodiment should
not be limited by any of the above described exemplary embodiments, but
should be defined only in accordance with the following claims and their
equivalents.